Evaluating sources and processing of nonpoint source nitrate in a small suburban watershed in China

Identifying nonpoint sources of nitrate has been a long-term challenge in mixed land-use watershed. In the present study, we combine dual nitrate isotope, runoff and stream water monitoring to elucidate the nonpoint nitrate sources across land use, and determine the relative importance of biogeochemical processes for nitrate export in a small suburban watershed, Longhongjian watershed, China. Our study suggested that NH₄⁺ fertilizer, soil NH₄⁺, litter fall and groundwater were the main nitrate sources in Longhongjian Stream. There were large changes in nitrate sources in response to season and land use. Runoff analysis illustrated that the tea plantation and forest areas contributed to a dominated proportion of the TN export. Spatial analysis illustrated that NO₃⁻ concentration was high in the tea plantation and forest areas, and δ¹⁵N-NO₃ and δ¹⁸O-NO₃ were enriched in the step ponds. Temporal analysis showed high NO₃⁻ level in spring, and nitrate isotopes were enriched in summer. Study as well showed that the step ponds played an important role in mitigating nitrate pollution. Nitrification and plant uptake were the significant biogeochemical processes contributing to the nitrogen transformation, and denitrification hardly occurred in the stream.